Materials from the extractive industry (mines, quarries, processing industries, etc.) are, when they are constituted by flammable organic materials, the cause of fires by self-heating related to the characteristics of these materials and explosions related to organic dusts brought about by such materials, and to the release of explosive gas as a result of self-heating during their storage, their transport and/or their handing.
Regarding limitation of dust emissions, various processes related to the protection of personnel as well as to respect for the environment have been developed.
Regarding limitation of explosion risks in the extractive industry of flammable organic materials, from products containing starch, document GB 1492238 can be cited, which describes, in the framework of coal mining operations from a coal seam, a method comprising an injection of product into holes drilled for the extraction of coal, in order to create a hydraulic fracture of the coal seam to partially eliminate the methane, oxidized by bacteria-based microbial action, and to moisten the dust, thereby reducing the dusts during the washing of coal. The aforementioned method uses a high viscosity aqueous composition comprising the indicated chemical products and starch. In addition, beyond the injection of the high viscosity product, the method in question adds compressed air, a solution of hydrochloric acid into the coal seam during the extraction, solution also heated preferably several hundreds of degrees, then a steam injection in order to remove the condensates.
In other words, an object of the aforementioned method is the elimination, in a heavy manner (high viscosity solution acting by bacterial action with additional injection of a solution of hydrochloric acid heated to several hundred degrees, then steam), the methane contained in the coal during the extraction, which has the effect of avoiding dusts during the extraction and washing.
Another document, WO9100866, describes, more particularly, the effect of an aqueous chemical composition designed to suppress the generation of dusts.
None of these documents addresses specific problems related to fire hazards caused by self-heating and the risk of explosion related to the fineness of the particles and more particularly microparticles of such materials, once extracted (dust explosion), with the risk of explosion caused by the release of explosive gas (carbon monoxide) as a result of self-heating (gas explosion) or with the risk of an explosion of a hybrid of gas and dust, during their storage, their transport and/or their handling.
On the other hand, for the prevention of such risks, various techniques for preventing and monitoring have been implemented without any of then, to date, having taken into account the fineness of particles of the materials and the binding of moisture by condensation which are the principal causes that can lead to a fire following a self-heating or an explosion of dust, gas, or hybrid (gas and dust).
The data enumerated below, which are necessary for the understanding of the scientific and experimental development that led to the discovery of unrecongnized characteristics related to concentrated steeping liquors, from the Final Report of the NERIS Convention CECA No. 7262/03/307/03 of 1996 entitled “Etude de sécurité incendie et explosion des installations houillères jour et sidérurgiques”, as well as data from this same organization relating to organic materials releasing flammable dusts.
Analysis of the Phenomenon of Ignition by Self-Heating of Extractive Organic Materials
Storage of coal and coke and, by extension: the organic materials of extractive origin, causes potential sources of fire resulting from self-heating of the products. This risk concerns the principal danger of this type of installation.
The phenomena of self-heating of coke and coal, essentially depend:                on the critical dimension of the storage that is in most cases exceeded;        on oxidation reactions taking place inside the product;        on the initial temperature;        on the binding of moisture.        
If the rates of oxidation of coal or of coke are more or less low as a function of their reactivity, they, however, play an important role for large storage dimensions where the critical dimension is greatly exceeded. Moreover, this oxidation rate which progresses with temperature elevation, also increases when the grain size of the materials in storage decreases, since the specific adjustable surface area increases sharply in this case. Furthermore, the binding of moisture by condensation in storage heavily promotes the elevation of the storage temperature, which accelerates the oxidation phemonena and of course self-heating. In fact, a condensation of the order of approximately 1% moisture induces a substantial increase in temperature of approximately 20° C.
Generally, the triggering of a fire, for most coals or cokes, requires an additional energy input in the form of minimum ignition energy in order to overcome the barrier of activation energy of the system. To have a combustion, it is also necessary to achieve the minimum temperature at which the reaction with flame development will occur. This temperature is called the ignition temperature.
With high molecular weight substances such as coal or coke, the decomposition must first occur well before reaching the ignition temperature. These phenomena take a certain time called the induction period. One speaks of ignition only if a flame is visible.
The induction period is reduced when the temperature increases. It also depends on the pressure, the combustible concentration, the type of material, thus its reactivity and its contained additives. The higher the sample volume, the easier the heat buildup.
In order to avoid the risks associated with self-heating, it is thus appropriate to store coal or coke in amounts below the critical volume for a given deposition temperature, or to work under inert atmosphere. In reality, these two possibilities are not realized, the critical dimension being generally low (a few meters) and storage under an inert atmosphere is not conceivable for storage in the open air.
The rate of oxidation of coals or cokes at ambiant temperature is very low and does not play an effective role in the phenomenon of self-heating except for heaps of large dimensions and after a very long time. This is the time required to pass from the storage temperature of the coal or coke (ambiant temperature) to a temperature of approximately 70-80° C. beyond which the self-heating takes place.
Moreover, the oxidation rate increases as the grain size decreases. In fact, when the particle size decreases, the specific surface of grains increases and thus also the surface subject to reacting with the oxygen in the air.
The pyrite content is also a factor influencing the rate of oxidation. The presence of pyrite in the form of well dispersed small particles (large specific surface) promotes the self-heating while the presence of monolythic pyrite does not cause substantial increase in the heating rate.
However, such a reaction can serve to prime heating because pyrite oxidizes rapidly only in very humid atmosphere. From 80° C., it is likely that the coal dries quickly but the rate of oxidation of the coal itself has now reached a sufficient value for heating by chemical oxidation to continue.
Hence the importance, beyond the reactivity of coal and/or coke itself, of the fineness of the particles of coal or coke and the binding of moisture by condensation in the phenomenon of ignition by self-heating.
Analysis of the phenomenon of explosion of dusts or hybrid (gas and dusts) emitted by extractive organic material
The explosion is a rapid combustion of a mixture of gas, coal dusts or both, with the air in a confined or partially confined space, wherein the heat released is greater than the heat lost in the environment.
In the industries associated with coal and similar materials, the explosions are associated with the presence of large quantities of dusts formed during the handling or the crushing of combustibles. Explosions can also be caused by pryolytic gas or incomplete combustion (carbon monoxide) formed during the self-heating of the coal deposit.
The energy released by a source of ignition primes the combustion of the particles nearby. Under the effect of this heat source, a pyrolysis of coal dusts then operates, generating volatile fuel that ignites. The reaction rates are high because the specific surface of the combustible is very substantial. If the cloud is sufficiently dense, the radiation and the convection heat the neighboring particles and the explosion thus propogates step by step.
The reactive mixture of air and coal particles “cold” (ambiant temperature) is transformed into combustion products “hot” (1200-2000° C.). The mixture of air particles that traverse the flame consequently undergoe a strong thermal expansion (the volume is multipled by at least 5 times). The expansion phenomenon is responsible for the observed pressure effects during an explosion in a confined or partially confined space.
In free atmosphere, in the case of stocks on the ground, only a fireball is observed with a very weak surge pressure, but with a large thermal effect. This is the “Flash”.
In closed chamber, the pressure effects are much larger (maximum surge pressure of approximately 5 to 10 bar) and can lead to destruction of equipment.
One speaks then of explosion.
The explosion is a particular combustion realizing the following specific parameters:                the dusts must be in suspension to form a large contact surface with air;        the dust concentration must be sufficient (greater than the minimum explosible concentration);        the atmosphere must be confined so that the pressure can increase.        
In order to eliminate any risk of explosion, it suffices to eliminate one of the components involved in the phenomenon of combustion or ignition.
Beyond the characterizations of combustion and ignition of dusts necessary for an explosion of organic dusts to be able to take place, various conditions concerning the dusts and their fineness, as well as the concentration and the combustion of these dusts in an atmosphere more or less confined and especially in an enclosed environment, must be met.
Hence the importance of the fineness of the particles of coal or coke in the explosion phenomenon.